This invention relates to an apparatus and process for measuring an optical characteristic. More specifically, this invention relates to an apparatus and process which allows rapid and easy placement of a series of similar articles in the same position below a meter for measuring the desired optical characteristic.
Food products and other consumer items are conventionally packed in printed wrappers in order that the products will make an attractive display on a supermarket shelf, or other retail display, thereby enhancing their attractiveness to their consumers. In order to achieve such attractiveness to consumers, the wrappers are frequently printed in several colors and may bear complex designs. For example, a wrapper intended for a cookie package may be color-printed with a picture of the cookies contained therein, while a wrapper intended for breakfast cereal may be decorated with a picture of the breakfast cereal in a bowl with milk and pieces of fruit.
The high-speed packaging machines used to pack mass-produced food and other consumer items may use hundreds of such wrappers per hour, and thus the wrappers are printed in very large batches with perhaps tens of thousands of individual wrappers in each batch. It is well known to those skilled in the art of printing such wrappers that, in the long printing runs required for multi-color printing of such large batches of wrappers, it is in practice impossible to keep the colors abolutely constant throughout the long printing run. Consequently, in order that the producers of food and other consumer items can produce a consistent product, such producers must constantly check the quality of the color printing on the wrappers they use and discard wrappers which differ too greatly from the desired colors.
Hitherto, decisions regarding the acceptability of the colors on wrappers have been made visually by personnel operating the packaging lines. The personnel are provided with samples showing the correct color and the limits of acceptable variation and simply compare the packages visually with these standards. Such subjective color comparisons have serious disadvantages. Subjective human color judgements are notoriously variable, especially when made by personnel who have little training and/or experience in color comparisons. Furthermore, the lighting in factories is often far from ideal for making such color comparisons, and an individual who is called to make such color comparisons throughout (say) an eight-hour shift will rapidly become fatigued. In addition, there are of course variations introduced by the physical and mental state of the individual, and futher inaccuracies introduced by the fact that packaging machines are frequently run 24 hours a day so that several different persons will be responsible for making the color comparisons on various shifts.
Instruments have now been developed which can measure a precise absolute value of specific colors, and store and process such information. These instruments provide an objective analysis of color quality in color-printed materials which is much less susceptible to variation than the subjective comparisons hitherto employed. Unfortunately, it is difficult to apply such instruments to control of color under the practical conditions in a packaging plant. Unlike the human eye, which can look at an entire wrapper having several different colors arranged in a large number of discrete areas in a design and make a succession of color comparisons of the individual colors with a set of standards lying adjacent the wrapper, the instruments must be brought closely adjacent a constant point on the succession of wrappers to be color tested. Not only may some of the color areas to be tested be small (as for example where a package is printed with a picture of cereal having pieces of fruit dispersed therein, and it is desired to carry out a color measurement on the pieces of fruit), but even where large areas of color are involved it is not sufficient merely to bring the instrument adjacent a large area of background color since it common practice to vary the intensity of a color within a large area covered by that color. Accordingly, it is not possible to rely upon an operator simply placing a measuring instrument manually adjacent a portion of the wrapper on which it is desired to measure the color, since invariably as the operator makes repeated measurements the position at which the instrument is placed adjacent the wrapper will vary, thereby giving faulty readings. Thus, in order to obtain reliable readings from such an instrument it is necessary to devise some form of locating device which will locate the instrument adjacent the same predetermined point on a series of wrappers.
Numerous forms of mechanical locating device have been described in the prior art. For example, U.S. Pat. No. 3184600 to Potter describes an apparatus for measuring coordinates in two dimensions. In this apparatus a stylus and sensor assembly is mounted on the end of a drafting table type arm so that it can move above a flat plate without rotation. A translucent plate is disposed above the stylus and sensor assembly parallel to the flat plane and is covered with grid lines. The stylus and sensor assembly comprises a stylus, two photocells and an upper plate. The upper plate has two separate halves, one of which is engraved with lines parallel to the X-axis of the lines on the translucent plate, while the other half of the plate is engraved with lines parallel to the Y-axis of the translucent plate. One photocell is placed below each half of the upper plate. As the two sets of lines on the upper plate move across the grid on the translucent plate, interference effects occur which enable the photocells in the stylus and sensor assembly to determine the X and Y coordinates of the stylus.
U.S. Pat No. 3950853 to Andrew describes an accessory for a track type drafting machine in which a drafting machine head comprising two rules disposed at right angles to one another is rotatably mounted on a carriage. This carriage is movable along a first track, which is itself moveable along a second track disposed at right angles to the first track. Consequently, by moving the drafting machine head along the first track, and the first track along the second track, the drafting head can be positioned at any position above a drawing board. Graduated tapes are provided on the tracks to enable the X and Y coordinates of the drafting head to be measured accurately.
U.S. Pat. No. 417645 to Copeland et al. describes a plotting apparatus in which a shaft is moveable along two slots extending perpendicular to the length of the shaft. A collar is slideable along the shaft so that by movement of the collar along the shaft, and of the shaft within the slots, the collar can move in two mutually perpendicular directions. A power-operated plotting arm including first and second arms is mounted on the collar. The power drive for the plotting arm pivots the first and second arms through controlled angles in response to the input of a pair of angular coordinate signals, thereby providing a high degree of resolution between points in the plotting operation.
Other prior art locating devices include apparatus for movement of a stylus or similar instrument in two dimensions by means of a pair of rigid rods disposed at right angles to one another. For example, U.S. Pat. No. 2847859 to Lynott describes such a positioning device. In the apparatus shown in FIG. 4 of this patent, four carriers are arranged to interact with an endless belt driven by a pulley. The four carriers are interconnected by two rods lying at right angles to one another and passing through sleeves also disposed at right angles to one another. This apparatus enables controlled positioning in an X-Y plane of a stylus or similar device attached to the two sleeves.
U.S. Pat. No. 2891314 to Haschek describes a map assembly including a location director which is mounted on two rods disposed at right angles to one another. Two pointers attached to the two rods indicate the X and Y coordinates of the location director on scales provided on two mutually perpendicular tracks disposed along two edges of the map. The patent suggests that a spotlight be provided on the location director to indicate a particular location on the map.
U.S. Pat. No. 2981123 to McHugh describes a coordinate positioner comprising a cutting block mounted on a first pair of smooth bearing shafts. A first threaded shaft is disposed between the first smooth shafts and extends through a threaded aperture in the cutting block, so that when the first threaded shaft is rotated the cutting block will slide in a precisely controlled manner along the smooth shafts. The ends of the two first smooth shafts and the first shaft are received in a pair of blocks, which are themselves slidable along a second pair of smooth shafts by means of a second threaded shaft.
U.S. Pat. No. 3241243 to Speer describes a hole center locating apparatus in which a contact sensing assembly can be translated in three dimensions. The apparatus disclosed in this patent operates in a manner generally similar to that in McHugh described above, but translates the contact sensing assembly in three dimensions using rack-and-pinion mechanisms, rather than the threaded shaft and collar mechanisms used in McHugh.
U.S. Pat. No. 3345747 to Sattler describes a mechanical coordinating device intended for use with machine tools. An apparatus shown in FIGS. 1-4 of this patent measures movement of a tool or other implement in a X-Y plane by causing photocells arranged to move with the implement to track across steel rules, thereby generating pulses when the photocells pass graduations cut into the rules. These impulses are electronically recorded as coordinates on appropriate digital counters.
U.S. Pat. No. 3495519 to Alfsen et al. describes an "XY table" which operates in a manner generally similar to the apparatus described in McHugh discussed above, except that the cutting block in McHugh is replaced with a plate supported upon a pair of spaced shafts. Like the apparatus shown in McHugh, the apparatus described in Alfsen effects movement of the plate in two perpendicular directions by means of threaded shafts passing through threaded collars.
U.S. Pat. No. 3665610 to Schlau et al. describes a plotter toy in which a pencil can moved in two dimensions across a sheet of paper by manual manipulation of a pair of knobs.
U.S. Pat. No. 3760505 to Clark describes a tracing device which is rather similar to the apparatus described in Haschek discussed above. In the Clark apparatus, a stylus is mounted upon a pair of rods disposed at right angles to one another, and these rods are moveable by means of a continuous loop of thread passing around pulleys.
U.S. Pat. No. 4397560 to Andresen describes an apparatus in which a magnetic locating device is used to locate a photocell at a plurality of positions on a rectangular grid. This apparatus is a photometer for sensing the optical density of a plurality of liquids located in a microtray provided with a rectilinear array of wells. The microtray is supported on a carrier having two hollow side members in which are enclosed elongate bar magnets disposed at right angles to one another. A light source is mounted on a cantilevered arm above the carrier, and a photocell is mounted on a support surface below the carrier. A light tube disposed below the light sources is provided with a detent which engages in the wells of the microtray. To enable the exact well being tested to be determined, two linear arrays of magnetic sensors are disposed beneath the support surface, these two linear arrays of sensors being disposed at right angles to one another. One of the sensors in each array is activated by the bar magnets, thereby enabling the exact well in the array of wells to be determined. Although the apparatus thus uses magnetic detection to detect which well is being tested, location of the wells is effected solely by the detent. Presumably, very accurate alignment of the wells is not necessary since the apparatus would normally be used for e.g. an ELISA assay, in which the whole area of any one well would be of the same color.
Other miscellaneous locating devices are described in the prior art. For example, U.S. Pat. No. 3491716 to Ranford describes a method for positioning a marker along two orthogonal axes which is somewhat similar to that in Haschek described above, except that instead of the marker being mounted on two rigid rods at right angles to one another it is moveable by means of a first cord extending along one axis. This cord is itself mounted by means of pulleys on a pair of shoes which slide along spaced tracks. Movement of the shoes along the tracks is controlled by means of a further pair of cords.
U.S. Pat. No. 3568321 to Maps et al. describes a plotting apparatus having a pin holding mechanism mounted for movement along an arm, which is itself moveable along opposed edges of a plotting surface.
Finally, U.S. Pat. No. 4237616 to Tobias describes a scanning path alignment apparatus including a pair of wires which are located one above the other in a plane that lies parallel or orthogonal to the center lines of a path which a scanning head is to view.
None of the prior art apparatus discussed above is very suitable for locating a color measuring instrument in a reproducible manner adjacent a predetermined point on a wrapper. Apparatus such as that described in McHugh, Speer, Sattler and Alfsen is bulky, complicated, and very expensive. Although such appratus is acceptable for use in costly numerically-controlled machine tools and similar devices, its bulk, complexity and cost renders it impractical for use in routine color determination of wrappers. The apparatus shown in Andresen is not capable of reproducibly locating a predetermined point with high precision. Most of the other prior art apparatus discussed above is intended for locating a point on a plane, or accurately measuring the location of such a point rather than merely reproducibly locating the same point on a series of sheets once that point has been located on the first sheet of the series. Consequently, such apparatus tends to be unnecessarily complicated and expensive for use in locating a measuring instrument adjacent a wrapper. Furthermore, in order to accommodate the need to place the instrument very closely adjacent the wrapper while effecting the color measurement, while still permitting rapid removal and replacement of wrappers, it is desirable that apparatus used in color measurement on wrappers allow for movement of the photocell toward and away from the plane of the wrapper. Most of the prior art apparatus described above does not permit movement of the stylus or other locating device perpendicular to the surface of the object on which the measurement is being effected. Finally, although an apparatus for effecting color measurements on wrappers only needs to locate an instrument at a single predetermined point for a given series of measurements, it must allow for variation in position in the predetermined point in two dimensions in order to accommodate variations in the printing of wrappers. This invention provides a simple, inexpensive apparatus which can meet all these requirements.